Channel State Information Reference Signals (CSI-RS) were introduced for LTE-Advanced (Long-Term Evolution) in Release 10 of the 3GPP (3rd Generation Partnership Project) specification. To use this feature, an eNB (Evolved Node-B) establishes a wireless communications channel with a UE (User Equipment), after which CSI-RS symbols are transmitted from the eNB to the UE. The UE performs channel state information measurements on the CSI-RS symbols in order to calculate channel state information. The UE then returns the channel state information to the eNB, in order to provide information to the eNB about the downlink signal quality for the wireless communications channel.
Meanwhile, Release 8 of the 3GPP specification provides for MIMO (Multiple Input Multiple Output) antenna configurations, and subsequent enhancements in Release 10 and Release 11 extend these provisions. MIMO may support beamforming to increase downlink channel quality. For Release 10, UEs may support 2-port antennas, 4-port antennas, or 8-port antennas. Under Release 13, UEs may support antennas comprising more ports, such as 12-port and 16-port antennas. Antennas comprising even greater numbers of ports are contemplated for Release 14, such as 32-port and 64-port antennas.
Through Release 11, the 3GPP specification is designed to support MIMO antenna configurations that are capable of adaptation in azimuth (i.e., a radial angle with respect to a reference angle in a horizontal plane). In future releases, however, the 3GPP specification may support FD-MIMO (Full-Dimensional MIMO) antenna configurations that are capable of adaptation in both azimuth and elevation.
Non-precoded CSI-RS may support FD-MIMO by facilitating channel state information measurement at the UE, which may in turn facilitate precoding selection for eNB antennas. At the same time, Release 13 will support the use of non-precoded CSI-RS with 12- and 16-port antennas.